M1 SP vs Air Gap

Based on my plug coloration and results, a square-jetted carb would run leaner in the lower plenum runners and chambers. A richer running cylinder would tend to be more carboned up but I would think ring seal would be more of a factor. Plug readings after a good 1/4 mile run, or after a few, and cut off before idling too much, which would contaminate the appearance, would be a better indicator.

It would seem that the lower lever would present a bigger plenum, which would reduce the vacuum signal and require slightly larger jets.